Isomerization-fractionation process

ABSTRACT

A process for separating isobutylene from C4 feeds by subjecting the feed to isomerization using a low-temperature active isomerization catalyst, reducing the butene-1 level followed by fractionation of the isomerized feed and again isomerizing the top fraction containing isobutylene and remaining butene-1 and isolating isobutylene therefrom. Preferably the isomerization of the feed is carried out in two stages under adiabatic conditions, with cooling in between.

United States Patent [1 1 Sprecher et al.

I l 1 l 1 3,758,604

[ 1 Sept. 11, 1973 ISOMERlZATION-FRACTIONATION PROCESS 75] lnvcntors: NathanVSprecher, Waterloo,

Belgium; John Thomas Cotter, Randolph, NJ.

[73] Assignee: Esso Research and Engineering Company, Linden, NJ.

[22] Filed: Mar. 15, 1972 [21] Appl. No.: 234,734

[30] Foreign Application Priority Data Mar. 19, 1971 Great Britain 7,309/71 [52] US. Cl 260/677 A, 260/683.2, 260/683.65 ['51] Int. Cl C07c ll/12 [58] Field of Search 260/677 A, 683.65,

[56] References Cited UNITED STATES PATENTS 3,284,535 1 H1966 Edwards et al. 260/677 A 8/1966 Halliwell 260/677 A 2/1966 Clay 260/677 A Primary ExaminerDelbert E. Gantz Assistant Examiner-Juanita M. Nelson Attorney-Leon Chasan et al;

57 ABSTRACT 13 Claims, 1 Drawing Figure l ISOMERIZATlON-FRACTIONATION PROCESS This invention relates to olefin isomerization and more specifically to double bond isomerization.

In one of its more specific aspects, the present invention relates to a continuous process of separating isobutylene from mixtures containing in addition butene-l.

There has now been developed an improved isomerization process, the principal purpose of which is double bond isomerisation to facilitate separation of the desired product. This process comprises a limited number of isomerization steps and accompanying separation steps, each of the isomerization steps being conducted under such conditions and with such catalysts as to maximize the desired type of double bond isomerization and optimize separation of the desired product in a substantially pure form.

More specifically, this invention contemplates a multistage isomerization-fractionation process where the isomerization steps are directed to isomerize hydrocarbons having boiling points near those of the desired product, to produce hydrocarbons having boiling points more removed from those of the desired product, hence facilitating the separation of the desired product from the hydrocarbon mixture. In one specific embodiment of this invention, it is desirable to obtain a substantially pure isoolefin from a mixture containing also one or more close boiling normal olefins, but separation by fractional distillation is difficult and expensive. Other methods like azeotropic distillation, solvent extraction and chemical separation do not appear as economical as the present invention; More specifically, for instance, in the case of isobutylene present in C hydrocarbon mixtures, such as C steam-cracked naphtha or gas oil cuts separation of isobutylene from butene-l is almost impossible, both compounds boiling within 0.3" C. It has been proposed in order to facilitate separation to introduce an intermediary isomerization step, by which butene-l is converted into butenes-2 of which the boiling point is sufficiently apart from isobutylene to make subsequent distillationfeasible. Such proposals, unless optimized, suffer from the drawback that several isomerization-distillation steps would be needed to produce isobutylene meeting the present stringest requirements of purity, which go to levels of below 0.1 percent of butene-l impurities.

The efficiency of the process depends very much on the quality of the isomerization catalyst, whichshould enable high conversion and high isomerization rate at low temperatures as well as-a long life-time. Even if such a catalyst is available, it is necessary to use the catalyst in a manner that will assure the most economic process which yields high purity isobutylene from various feeds and thus requires optimization regarding the number of isomerization and distillation steps.

It is an important object of the present invention to provide an economic continuous process for separating isobutylene of'high purity from various feeds, especially C, feeds, using a low-temperature active isomerization catalyst with a minimum of isomerization and fractionation stages.

It is a further object of the invention to provide such a process wherein the isomerization and fractionation steps are separated from each other thereby allowing independent and optimum" performance, from each step.

It is another object of the invention to provide a process for separating isobutylene from various feeds wherein feed fluctuations are tolerated and do not affect the performance of the process.

-It is another object of the present invention to provide such a process wherein the isomerization steps may be conducted in either vapor or liquid phase operation at temperatures from about -50 C to about 200 C, at space velocities from about 0.4 to about 40 liquid volumes of olefin per volume of catalyst per hour, at pressures from about atmospheric toabout 250 psig.

Preferably, this invention contemplates double bond isomerization reactions which are carried out at relatively low temperatures conditions, which are favourable to obtain a low butene-l level according to the thermodynamic equilibrium. Catalysts of high isomerization activity are generally applicable. Thus lowtemperature active isomerization catalysts are preferred which are active at temperatures below 200C,

preferably below 100C in the range of -50 to +100C.

Anfexample of such a catalyst has been described in our copending application Ser. No. 54,162/; the disclosure from said application is herein incorporated by reference.

Accordingly, the invention provides a continuous process for separating isobutylene from C,-mixtures containing butene-l insubstantial proportions, which process comprises the following steps:

a'. isomerizing the C,-mixture by contacting it with an isomerization catalyst which converts butene-l into butenes-2 at a temperature in the range of 50 to 200C, in such a way and under such cooling conditions that at least during part of the isomerization reaction the C -mixture is contacted with a low-temperature active isomerization catalyst at temperatures below 200C to obtain an isomerized mixture with a butene-l level of at most 3 wt.'% of the initial n C -olefin content,

b. fractionating the isomerized blend into a top fraction comprising isobutylene and the remaining butene-l and a bottom fraction comprising butenes-2 .and other high-boiling compounds,

c. isomerizing the top fraction with .a lowtemperature activeisomerization catalyst at a temperature below 100C to reduce still further the butene-l level,

d. recirculating a substantial top fraction and v I e..subjecting the remaining part of the isomerized top fraction to further fractionation to isolate isobutyl- "ene containing less than'l wt.% butene-l.

l n a preferred modification of the process. according to the inventionthe initial isomerization is carried-out in two steps, the first step under such-conditions that the butene-l level is substantially reduced and the second step under more favourable'conditions to reduce the butene-l level as far as possible, preferably below 5 or 4 percent from the butene-l content of the effluent' of the first reactor.

The catalysts employed in both steps need not be identical. it is possible to use in the first step a catalyst which is sufficiently active to lower the butene-l level to a value ,in the range of 5 15 percent of the original part of the isomerized content. With a'suitable catalyst this reaction may be conducted a wide temperature range from -50 C to +200 C. Conveniently, however, is a range from 0 C C; Preferably, however, the catalysts employed in both steps are identical and only the temperatures of both steps are different, higher in the first one.

The two-step isomerization process offers the advantage that one is less dependant on fluctuations in the catalyst and feed composition and heat of reaction is more easy removed. The isomerizations can be carried out in one vessel with temperature control but it is preferred to carry them out in two separate vessels separated by a heat-exchanger. The isomerization is exothermic and heat of reaction released will raise the temperature and affect the performance of the catalyst. Increasing temperature directionally limits the equilibrium butene-l conversion. Thus it is preferred to run the two stages substantially adiabatically with cooling between them. Compared with the lowest level of butene-l possible under the most favourable reaction conditions using a low-temperature active catalyst, the first stage will preferably be run in such a way that substantially 90 100 percent of the isomerization (compared with the thermodynamic equilibrium values) and correspondingly of the heat generation will occur. The finishing operation will be conducted in the second stage where only a moderate temperature increase will be allowed, of no more than say C. The two-stage initial isomerization provides an efficient way to reach the very high purity requirements which are at present demanded, that is the benefit of the invention will be especially obtained if butene-l levels of lower than 3 percent, down to 0.1 percent and even lower are desired.

It also offers the advantages that the temperature in the first stage can be increased, if necessary, to. compensate for declining catalyst activity with time, provides a buffer zone to catch possible catalyst poisons and allows for flexibility in operation (spare first stage may be provided). After the isomerization has been completed at low temperature the mixture which contains less than 5 percent of the original butene-l content (preferably less than 4 percent) is fed into a fractionation tower operating at such a temperature that the feed is split into a top fraction containing the isobutylene and traces of butene-l and a bottom fraction containing the butenes-Z and other compounds.

It is preferred to carry out recirculation in combination with the second isomerization to keep thenumber of fractionation units to be absolute minimum.

Dependant on the final specification required the recirculation ratio will be adjusted. The isomerization of the top fraction is conveniently carried out at low temperatures with a low temperature active catalyst to ob-' tain optimal results.

The process is also applicable to feeds containing beside isobutylene and butene-l, isobutane and butadiene.

A separate fractionating step will be included after the initial isomerization and fractionation to remove the afore-mentioned components. Their presence during the initial isomerization and fractionation steps does not affect the efficiency of the process. The composition of the feed, in particular, the ratio of isobutylem: to butene-l does not have an effect on the efficiency of the process. It is an advantage that in principle any feed with varying ratios of isobutylene to bu,- tene-i can be handled. For instance, refinery stream containing usually 50 wt of butene-l in combination with isobutylene which may vary in the same range can be treated according to the process of the inwere as follows:

vention to separate an isobutylene stream containing less than 3 percent, and even less than 0.1 percent of butene-l.

The process of the invention will be more easily understood when explained in reference to the accompanying drawing in which one specific embodiment of the process is schematically represented. The isomerization step will be described as a liquid phase operation but could as well be operated in the vapor phase.

The feed, a C blend, is introduced at 1 and pumped by means of pump 2 in the first isomerization reactor 3. The reactor is filled with the isomerization catalyst (in this case a solid, heterogeneous catalyst) and operated at temperatures in the. range of -50 C to+200C but preferably in the range of 0 C C. This temperature as well as the throughput is adjusted to isomerize a substantial part of the butene-l, that is up to the equilibrium position at the given temperature, which is not the optimal one reachable with the lowtemperature active catalyst. Heat exchanger 4 cools the effluent of reactor 3 to the desired low temperature for carrying out the second isomerization in reactor 5. The isomerized stream is then introduced through pipes 6 and 7 into the fractionator 8. The cis and trans butenes- 2 are removed in the bottom fraction at 9 and the top fraction 10 containing mainly isobutylene. and butene-l is cooled by cooler 11, condensed in condensor 12 and subjected to isomerization in the reactor 13. Reactor 14 is a spare reactor which can be used if, for instance, the reactor 13 has operated a long time and the catalyst activity has decreased. The effluent ofreactor 13 is partly recirculated through pipe 15 which is connected to pipe 7 to the fractionation tower 8. The other part is fed through pipe 16 to fractionator 17 where isobutane is removed as the top-product at 18. The heavier fraction containing isobutylene is fed by pipe 19 into fractionator 20, where 1 purified isobutylene is withdrawn at 21. The heavy ends can be recirculated to fractionator 8 if desired (through pipe 22).

The following table illustrates the results with a par ticular C stream, having a typical C stream-cracked naphtha composition, after butadiene extraction.

The catalyst consisted of. cobalt-lI-acetylacetonate, supported on Si0 and reduced by triisobutylaluminium, preferably in the presence of the olefins, as described in our copending Ser. No. application 54,162/70. Another possible catalyst is sodium on alumina. A pure butene-2 stream is available at 9, which is essentially free of isobutylene and butene-l'.

The isomerization conditions in the three reactors Iso-l Reactor 1 5215151 -2 Reactor Stage 1 Stage 2 Stage 3" Temperature ZO C KO C fi fi C- 45C 38C. SpaceVelocity l7 VVH l7 VVH l3 VVH The fractionation temperature may be in the range of 40 C 150 C. and in this example was 65 C.

v -Weight percent Stream No 1 5 10 15 7 19 21 IsoC1 9.1 9.1v 23.5 23.5 15.1 1.8 1.8 29.0 29.0 14.3 74.3 48.4 95.2 98.0 37.9 2.4 1.5 0.054 1.1 0.08 0.08 2.7 2.7 0.1 0.1 1.1 0.10 0.10 14.4 42.5 0.45 1.47 24.8 1.86 0.05 0.9 14.5 0.005 0.51 9.1 0.68 0.001

What is claimed is:

l. A continuous process for separating isobutylene from C mixtures containing butene-l, comprising the following steps:

a. isomerizing the C, mixture by contacting it with an isomerization catalyst which converts butene-l into butenes-2 at a temperature in the range of -50 to 200C, in such a way and under such cooling conditions that at least during part of the isomerization reaction the C,-mixture is contacted with a low temperature active isomerization catalyst at temperatures below 100C to obtain an isomerized mixture with a butene-l level of at most 3 wt.% 0 the initial :1 C,-olefin content,

b. fractionating the isomerized blend into a top fraction comprising isobutylene and the remaining butene-l and a bottom fraction comprising butenes-Z and other high boiling compounds,

isomerizing the top fraction with a lowtemperature active isomerization catalyst at a temperature below 100C to reduce still further the butene-l level,

d. recirculating a substantial part of the isomerized top fraction and e. subjecting the remaining part of the isomerized top fraction to further fractionation to isolate isobutylene containing less than 1 wt.% butene-l.

2. A continuous process according to claim 1, wherein the isomerization of the C,-mixture is carried out in two steps and in the first step the major amount of butene-l is isomerized to butenes-2.

3. A continuous process according to claim 2, wherein 90 100 percent of the isomerization (compared with the thermo-dynamic equilibrium values) is carried out in the first step.

4. A continuous process according to claim 2, wherein the isomerization is carried out in two separate vessels separated by a heat-exchanger.

5. A continuous process according to claim 4, wherein the isomerizations are carried out under adiabatic conditions with cooling in between.

6. A continuous process according to claim 2, wherein the temperature rise in the second step is no more than 10C.

7. A continuous process according to claim 2, wherein the isomerization in the second step is carried out at a temperature in the range of 0 to 75C.

8. A continuous process according to claim 1, wherein the C,-feed mixture contains isobutylene and- /or b'utadiene, which are separated by fractionation after the isomerization.

9. A continuous process according to claim 1, wherein the isomerization steps are conducted in either vapour or liquid phase operation at temperatures from about 50 to about 200C, at space velocities from about 0.4 to about 40 liquid volumes of olefin per volume of catalyst per hour, at pressures from about atmospheric to about 250 psig.

10. A process according to claim 1, wherein an is'om erization catalyst is employed which is active at a temperature below 100C.

11. A process according to claim 1, wherein a C feed is isomerized in a first step in the presence of a catalyst in such a way that the butene-l level is reduced to a value of 5 15 percent of the original content, cooling the isomerized feed to remove heat of reaction and subjecting the cooled isomerized feed to a second isomerization in thepresence of a catalyst active at low temperatures, the reaction temperature in the second step being in the range of O to C, both steps being carried out under adiabatic conditions, feeding the mixture containing less than 5 percent of vthe original butene-l content in a fractionation tower to recover as a top fraction isobutylene plus traces of butene-l and as a bottom fraction the butenes-Z and other compounds, isomerizing the top fraction at low temperatures in the range of 0 to 75C in the presence of a lowtemperature active catalyst,.recirculating part of the isomerized top fraction and recovering substantially pure isobutylene by further fractionation.

12. The process according to claim 1 wherein the catalyst is cobalt ll acetyl acetonate supported on Si0 and reduced by an alkyl alumina.

13. The process according to claim 1 wherein the catalyst is sodium on alumina.

* a a I 

2. A continuous process according to claim 1, wherein the isomerization of the C4-mixture is carried out in two steps and in the first step the major amount of butene-1 is isomerized to butenes-2.
 3. A continuous process according to claim 2, wherein 90 - 100 percent of the isomerization (compared with the thermo-dynamic equilibrium values) is carried out in the first step.
 4. A continuous process according to claim 2, wherein the isomerization is carried out in two separate vessels separated by a heat-exchanger.
 5. A continuous process according to claim 4, wherein the isomerizations are carried out under adiabatic conditions with cooling in between.
 6. A continuous process according to claim 2, wherein the temperature rise in the second step is no more than 10*C.
 7. A continuous process according to claim 2, wherein the isomerization in the second step is carried out at a temperature in the range of 0 to 75*C.
 8. A continuous process according to claim 1, wherein the C4-feed mixture contains isobutylene and/or butadiene, which are separated by fractionation after the isomerization.
 9. A continuous process according to claim 1, wherein the isomerization steps are conducted in either vapour or liquid phase operation at temperatures from about -50* to about 200*C, at space velocities from about 0.4 to about 40 liquid volumes of olefin per volume of catalyst per hour, at pressures from about atmospheric to about 250 psig.
 10. A process according to claim 1, wherein an isomerization catalyst is employed which is active at a temperature below 100*C.
 11. A process according to claim 1, wherein a C4-feed is isomerized in a first step in the presence of a catalyst in such a way that the butene-1 level is reduced to a value of 5 - 15 percent of the original content, cooling the isomerized feed to remove heat of reaction and subjecting the cooled isomerized feed to a second isomerization in the presence of a catalyst active at low temperatures, the reaction temperature in the second step being in the range of 0 to 75*C, both steps being carried out under adiabatic conditions, feeding the mixture containing less than 5 percent of the original butene-1 content in a fractionation tower to recover as a top fraction isobutylene plus traces of butene-1 and as a bottom fraction the butenes-2 and other compounds, isomerizing the top fraction at low temperatures in the range of 0 to 75*C in the presence of a low-temperature active catalyst, recirculating part of the isomerized top fraction and recovering substantially pure isobutylene by further fractionation.
 12. The process according to claim 1 wherein the catalyst is cobalt II acetyl acetonate supported on Si02 and reduced by an alkyl alumina.
 13. The process according to claim 1 wherein the catalyst is sodium on alumina. 